Photoinduced Oxidation of Water to Oxygen in the Ionic Liquid

Mar 10, 2009 - Liquid BMIMBF4 as the Counter Reaction in the Fabrication of ... or deliberately added water present in the ionic liquid as the sacrifi...
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Photoinduced Oxidation of Water to Oxygen in the Ionic Liquid BMIMBF4 as the Counter Reaction in the Fabrication of Exceptionally Long Semiconducting Silver-Tetracyanoquinodimethane Nanowires Chuan Zhao and Alan M. Bond* School of Chemistry and ARC Special Research Center for Green Chemistry, Monash UniVersity, Clayton, Victoria 3800, Australia Received August 31, 2008; E-mail: [email protected]

Abstract: The synthesis of exceptionally long semiconducting silver tetracyanoquinodimethane (AgTCNQ) nanowires has been achieved in the room temperature ionic liquid, 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4) by photochemical reduction of TCNQ to TCNQ-, coupled with use of benzyl alcohol as the sacrificial electron donor. The presence of Ag(I) in the ionic liquid allows formation of mm length AgTCNQ nanowires onto both conducting and insulating surfaces, via a nucleation and diffusioncontrolled growth mechanism. Remarkably, photocrystallization also can be achieved using adventitious or deliberately added water present in the ionic liquid as the sacrificial electron donor. In this case, oxidation of water produces O2 as the counter reaction in the photoreduction of TCNQ. In contrast, irradiation in “dried” ionic liquids fails to induce any detectable photochemistry. Molecular structural differences, relative to the situation encountered in more conventional solvent media, are believed to account for the more favorable kinetics available for oxidization of water in ionic liquids. Water assisted photocrystallization in viscous BMIMBF4 where diffusion rate are slow leads to slow growth of AgTCNQ, thereby allowing easy manipulation of AgTCNQ morphology from small nanoparticles to extremely long single nanowires by tuning the irradiation and ripening time. Infrared and Raman spectroscopic data, elemental analysis, and scanning electron microscopic images all confirm the formation of highly pure AgTCNQ nanomaterials. This study highlights the significant role that water present as an adventitious impurity may play in photochemical studies in ionic liquids and also suggests that ionic liquids may provide a favorable environment for photochemically based water splitting.

Introduction

Semiconducting metal-7,7,8,8,-tetracyanoquinodimethane (TCNQ) materials have been of considerable interest since the 1970s, owing to their novel electrical and/or magnetic properties.1-14 To date, applications have been reported in (1) Kepler, R. G.; Bierstedt, P. E.; Merrifield, R. E. Phys. ReV. Lett. 1960, 5, 503. (2) Melby, L. R.; Harder, R. J.; Hertler, W. R.; Mahler, W.; Benson, R. E.; Mochel, W. E. J. Am. Chem. Soc. 1962, 84, 3374. (3) Torrance, J. B.; Scott, B. A.; Kaufman, F. B. Solid State Commun. 1975, 17, 1369. (4) Potember, R. S.; Poehler, T. O.; Cowan, D. O. Appl. Phys. Lett. 1979, 34, 405. (5) Potember, R. S.; Poehler, T. O.; Cowan, D. O.; Bloch, A. N. NATO ASI Ser., Ser. C 1980, 56, 419. (6) Kathirgamanathan, P.; Rosseinsky, D. R. J. Chem. Soc., Chem. Commun. 1980, 839. (7) Potember, R. S.; Poehler, T. O.; Benson, R. C. Appl. Phys. Lett. 1982, 41, 548. (8) Kamitsos, E. I.; Risen, J. W. M. Solid State Commun. 1983, 45, 165. (9) Benson, R. C.; Hoffman, R. C.; Potember, R. S.; Bourkoff, E.; Poehler, T. O. Appl. Phys. Lett. 1983, 42, 855. (10) Poehler, T. O.; Potember, R. S.; Hoffman, R.; Benson, R. C. Mol. Cryst. Liq. Cryst. 1984, 107, 91. (11) Duan, H.; Cowan, D. O.; Kruger, J. J. Electrochem. Soc. 1993, 140, 2807. (12) Gu, N.; Yang, X.-M.; Sheng, H.-Y.; Lu, W.; Wei, Y. Synth. Met. 1995, 71, 2221. 10.1021/ja806893t CCC: $40.75  2009 American Chemical Society

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behavior and memory effects available with both AgTCNQ and CuTCNQ has stimulated considerable renewed interest of these materials in nonvolatile-based memory devices.29-44 Complementary metal oxide semiconductor (CMOS) devices fabricated from these TCNQ materials have been downscaled in size to an area of 0.25 µm2.41 One-dimensional (1D) nanostructures generallyfavorsizereductionneededinthisareaoftechnology.44-47 In this context, it has been found that AgTCNQ nanowires can be used in high density integration of nanodevices.48,49 Importantly they have a much higher on-off ratio than thin films.48 Many AgTCNQ preparation procedures are based on reaction of TCNQ dissolved in acetonitrile with metallic Ag. In these so-called “spontaneous electrolysis” methods, metallic Ag is oxidized to the Ag+ cation and TCNQ reduced to the TCNQradical anions to give AgTCNQ at the metal surface.30,38,39,50 Morphology and thickness of the AgTCNQ films can be modulated by changing the experimental conditions to yield microwires, needle-shaped crystals and also dendrites.38,39,50 In addition to these “wet” methods, AgTCNQ films also can be prepared by “dry” procedures, such as chemical vapor deposition of TCNQ onto Ag metal surfaces8,49 and thermal codeposition of TCNQ and Ag.49 Synthetic methods derived from TCNQ and Ag salt precursors also have been widely used.2,14 Early studies based on electrocrystallization of AgTCNQ employed oxidation of Ag substrate electrodes in acetonitrile solutions containing a TCNQ- (LiTCNQ) salt.6,51 Electrochemical methods based on solid-solid interconversion at an aqueous TCNQ electrode interface, or reduction of TCNQ in the presence of (29) Yamaguchi, S.; Viands, C. A.; Potember, R. S. J. Vac. Sci. Technol., B 1991, 9, 1129. (30) Heintz, R. A.; Zhao, H.; Ouyang, X.; Grandinetti, G.; Cowen, J.; Dunbar, K. R. Inorg. Chem. 1999, 38, 144. (31) Neufeld, A. K.; Madsen, I.; Bond, A. M.; Hogan, C. F. Chem. Mater. 2003, 15, 3573. (32) Fan, Z.; Mo, X.; Lou, C.; Yao, Y.; Wang, D.; Chen, G.; Lu, J. IEEE Trans. Nanotechnol. 2005, 4, 238. (33) Oyamada, T.; Tanaka, H.; Matsushige, K.; Sasabe, H.; Adachi, C. Appl. Phys. Lett. 2003, 83, 1252. (34) Neufeld, A. K.; O’Mullane, A. P.; Bond, A. M. J. Am. Chem. Soc. 2005, 127, 13846. (35) Liu, H.; Zhao, Q.; Li, Y.; Liu, Y.; Lu, F.; Zhuang, J.; Wang, S.; Jiang, L.; Zhu, D.; Yu, D.; Chi, L. J. Am. Chem. Soc. 2005, 127, 1120. (36) Harris, A. R.; Neufeld, A. K.; O’Mullane, A. P.; Bond, A. M.; Morrison, R. J. S. J. Electrochem. Soc. 2005, 152, C577. (37) Cao, G.; Ye, C.; Fang, F.; Xing, X.; Xu, H.; Sun, D.; Chen, G. Micron 2005, 36, 267. (38) Cao, G.; Ye, C.; Fang, F.; Xing, X.; Xu, H.; Sun, D.; Chen, G. Mater. Sci. Eng., B 2005, 119, 41. (39) Liu, Y. L.; Ji, Z. Y.; Tang, Q. X.; Jiang, L.; Li, H. X.; He, M.; Hu, W. P.; Zhang, D. Q.; Wang, X. K.; Wang, C.; Liu, Y. Q.; Zhu, D. B. AdV. Mater. 2005, 17, 2953. (40) Muller, R.; Genoe, J.; Heremans, P. Appl. Phys. Lett. 2006, 88, 103501. (41) Muller, R.; De Jonge, S.; Myny, K.; Wouters, D. J.; Genoe, J.; Heremans, P. Solid-State Electron. 2006, 50, 602. (42) O’Mullane, A. P.; Fay, N.; Nafady, A.; Bond, A. M. J. Am. Chem. Soc. 2007, 129, 2066. (43) Zhou, W.; Ren, L.; Lin, F.; Jiao, L.; Xue, T.; Xian, X.; Liu, Z. Appl. Phys. Lett. 2008, 93, 123115. (44) Xiao, K.; Tao, J.; Pan, Z.; Puretzky, A. A.; Ivanov, I. N.; Pennycook, S. J.; Geohegan, D. B. Angew. Chem., Int. Ed. 2007, 46, 2650. (45) Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. (46) Wang, Z. L. AdV. Mater. 2002, 12, 1295. (47) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (48) Fan, Z.; Lu, J.; Mo, X.; Chen, G. ReV. AdV. Mater. Sci. 2003, 4, 36. (49) Ye, C. N.; Cao, G. Y.; Mo, X. L.; Fang, F.; Xing, X. Y.; Chen, G. R.; Sun, D. L. Chin. Phys. Lett. 2004, 21, 1787. (50) Cao, G.; Fang, F.; Ye, C.; Xing, X.; Xu, H.; Sun, D.; Chen, G. Micron 2005, 36, 285. (51) Shields, L. J. Chem. Soc., Faraday Trans. 2 1985, 81, 1. 4280

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Ag salts also have been used.52 Recently, O’Mullane et al. developed a photochemical method which takes advantage of photoinduced electron-transfer between TCNQ (electron acceptor) and a sacrificial electron donor (benzyl alcohol) to generate TCNQ-, which then reacts with metal cations such as Ag+ dissolved in the acetonitrile solution, to form metal-TCNQ materials.42 The morphology of the photocrystallized products depends on the cation and ranges from microrods to nanowires. In the past decade, use of ionic liquids (ILs), as “green” replacements for conventional organic solvents in chemical processes has received much attention.53-55 Ionic liquids provide an attractive medium for synthesis because of properties such as (i) large electrochemical potential windows, which provide access to electrodeposition of metals and semicoductors not possible from conventional aqueous or organic electrolytes;55-58 (ii) low interfacial tension, which makes them easy to adapt to other phases and results in high nucleation rates and generally smaller nanoparticles;59 (iii) high thermal stability, which allows syntheses to be conducted at temperatures above 100 °C without using high-pressure vessels; (iv) functionalized ILs, which can modify material surfaces and control the size and improve the stability of nanoparticles;60,61 (v) highly structured hydrogen bonding, which can provide templates for formation of welldefined and extended nanostructures.59,62 Recognition of these properties has led to synthesis of metal nanoparticles,63,64 TiO2 nanospheres65 and hollow microspheres,66 Te nanorods and nanowires,67 Sb2S3 and Bi2S3 nanorods,68 CoPt nanowires,69 ZnO flowers,70 and porous silica sponge,71 and electrodeposition of nanostructured metals,56-58 semiconductors and conducting polymers.56,58,72 A few recent reports also have described the use of ionic liquids as a medium for photochemical reactions73 and for synthesis of Au nanoparticles.74,75 In this report, we describe, for the first time, the application of ionic liquid as a medium for synthesis of charge transfer (52) Harris, A. R.; Nafady, A.; O’Mullane, A. P.; Bond, A. M. Chem. Mater. 2007, 19, 5499. (53) Rogers, R. D., Seddon, K. R.,Eds. Ionic Liquids: Industrial Applications to Green Chemistry; ACS Symposium Series 818; American Chemical Society: Washington, DC, 2002. (54) Rogers, R. D., Seddon, K. R., Volkov, S., Eds. Green Industrial Applications of Ionic Liquids; Kluwer Academic: Dordrecht, Netherlands, 2003. (55) Zhao, C.; Burrell, G.; Torriero, A. A. J.; Separovic, F.; Dunlop, N. F.; MacFarlane, D.; Bond, A. M. J. Phys. Chem. B 2008, 112, 6923. (56) Endres, F. ChemPhysChem. 2002, 3, 144. (57) Silverster, D. S.; Compton, R. G. Z. Phys. Chem. 2006, 220, 1247. (58) Endres, F.; MacFarlane, D.; Abbott, A. Electrodeposition from Ionic Liquids; John Wiley & Sons: Hoboken, NJ, 2008. (59) Antonietti, M.; Kuang, D.; Smarsly, B.; Zhou, Y. Angew. Chem., Int. Ed. 2004, 43, 4988. (60) Fei, Z.; Geldbach, T. J.; Zhao, D.; Dyson, P. J. Chem.-Eur. J. 2006, 12, 2122. (61) Itoh, H.; Naka, K.; Chujo, Y. J. Am. Chem. Soc. 2004, 126, 3026. (62) Mele, A.; Tran, C. D.; De Paoli Lacerda, S. H. Angew. Chem., Int. Ed. 2003, 42, 4364. (63) Dupont, J.; Fonseca, G. S.; Umpierre, A. P.; Fichtner, P. F. P.; Teixeira, S. R. J. Am. Chem. Soc. 2002, 124, 4228. (64) Fonseca, G. S.; Umpierre, A. P.; Fichtner, P. F. P.; Teixeira, S. R.; Dupont, J. Chem.-Eur. J. 2003, 9, 3263. (65) Zhou, Y.; Antonietti, M. J. Am. Chem. Soc. 2003, 125, 14960. (66) Yoo, K.; Choi, H.; Dionysiou, D. D. Chem. Commun. 2004, 2000. (67) Zhu, Y. J.; Wang, W. W.; Qi, R. J.; Hu, X. L. Angew. Chem., Int. Ed. 2004, 43, 1410. (68) Zhu, Y. J.; Jiang, Y. J. Phys. Chem. B 2005, 109, 4361. (69) Wang, Y.; Yang, H. J. Am. Chem. Soc. 2005, 127, 5316. (70) Wang, W. W.; Zhu, Y. J. Inorg. Chem. Commun. 2004, 7, 1003. (71) Zhou, Y.; Antonietti, M. Nano Lett. 2004, 4, 477. (72) Pringle, J. M.; Efthimiadis, J.; Howlett, P. C.; Efthimiadis, J.; MacFarlane, D. R.; Chaplin, A. B.; Hall, S. B.; Officer, D. L.; Wallace, G. G.; Forsyth, M. Polymer 2004, 45, 1447.

Photoinduced Oxidation of Water to Oxygen

metal-TCNQ complexes by photocrystallization. Parameters that affect the photocrystallization and growth processes, such as sacrificial electron donor, irradiation time, ripening time and water content present in the ionic liquid have been investigated in detail. Significantly, the study leads to the surprising discovery that water in the ionic liquid BMIMBF4, present either as an adventitious impurity or deliberately added, can be photooxidized to oxygen and hence serve as a sacrificial electron donor in this kind of environment. Thus BMIMBF4 provides a favorable medium for photooxidation of water as well as for synthesizing technologically important metal-TCNQ materials with nanowire morphologies. Experimental Section Materials and Chemicals. Ionic liquid, 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4) from Merck (high purity grade, g99.0%, batch code 4.91049.0100), 98% tetrakis(acetonitrile) silver(I) tetrafluoroborate (Ag(MeCN)4BF4) and silver(I) tetrafluoroborate (AgBF4) from Sigma, 98% 7,7,8,8,-tetracyanoquinodimethane (TCNQ), absolute ethanol and benzyl alcohol from Aldrich and acetonitrile from Omnisolv were used as provided by the manufacturer. Photochemical Procedures. Initially, an irradiation procedure described for photocrystallization in acetonitrile was modified for ionic liquid applications42 under benchtop laboratory conditions. Subsequently, procedures were repeated on samples of known water content in a nitrogen filled glovebox (see details below). Typically, 4.5 mM Ag(MeCN)4BF4 or AgBF4 and 4.5 mM TCNQ were dissolved in BMIMBF4 using 3 min sonication, if required. The sacrificial electron donor benzyl alcohol was added at a 10% v/v concentration. The ionic liquid solution was then drop cast onto an indium tin oxide (ITO) or glass substrate (2.5 × 2.5 cm2) to give a liquid drop, or a thin liquid film. A thin glass rod was used to spread the drop. Owing to the very low vapor pressure of BMIMBF4, liquid drops or thin films do not evaporate, even over the long duration of some experiments (up to 1 week). The ionic liquid drop or thin film are then irradiated from above at a distance of 0.5 cm using an optical fiber attached to a light source (Newport Scientific, Model 6292 200 W Hg(Xe) lamp), typically for 30 min. The irradiation generated a ca. 3.2 mm diameter blue spot associated with the formation of AgTCNQ. Due to the high viscosity of BMIMBF4, the blue AgTCNQ crystals formed in the liquid drop do not precipitate onto the substrate surface. Ten microliters of ionic liquid was then removed from the middle of the blue spot containing AgTCNQ and redrop cast onto a 2.5 cm diameter membrane filter having a pore size of 0.1 µm (Super 100, Gelman Science, MI). This substrate facilitates ionic liquid removal and hence rapid precipitation of AgTCNQ onto the membrane surface. This substrate is particularly suitable for isolating materials having relatively large structures (at least in one dimension), as small structures may be rinsed through the membrane pores. Alternatively, microscopic glass slides (2.5 × 2.5 cm2) were used as the substrate for drop recasting. In this case, the substrate with the blue ionic liquid drop was stored in the dark overnight to allow precipitation of AgTCNQ to occur onto the glass surface. The ionic liquid was then removed with a tissue (Kimwipe). Next, the substrate was gently immersed into water bath for 10 min to allow complete removal (by dissolution) of BMIMBF4 and then gently rinsed by adding drops of acetonitrile and ethanol, which were then removed with a tissue. Finally, the substrate was dried under a gentle N2 stream. To obtain larger quantities of AgTCNQ, the above procedure was adapted to a specimen tube (12 mm diameter, SAMCO) filled (73) Alvaro, M.; Ferrer, B.; Garcia, H.; Narayana, M. Chem. Phys. Lett. 2002, 362, 435. (74) Firestone, M. A.; Dietz, M. L.; Seifert, S.; Trasobares, S.; Miller, D. J.; Zaluzec, N. J. Small 2005, 1, 754. (75) Zhu, J.; Shen, Y.; Xie, A.; Qiu, L.; Zhang, Q.; Zhang, S. J. Phys. Chem. C 2007, 111, 7629.

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with ionic liquid solution which was irradiated at λ ) 425 nm (from below at a distance of approximately 4 cm) with a Polilight PL6 xenon lamp (300 W, Rofin Australia) fitted with a light guide cable. Water was then added to the specimen tube to reduce the viscosity and AgTCNQ material was collected by centrifugation after washing 3 times sequentially with acetonitrile and ethanol, and finally drying under vacuum. Detection of Benzaldehyde. The course of benzaldehyde generation from benzyl alcohol was monitored by employing a Varian 3700 gas chromatograph and HP3396 integrator with a 6′ × 1/4” Carbowax 20 M column. Its identity was confirmed by comparison of the retention time of the peak with that found using an authentic sample of benzaldehyde, and the concentration quantified by standard addition. Further details are available in reference.76 Electrochemical Detection of O2. The photochemical experiments with voltammetric detection of oxygen (and TCNQ-) were undertaken in a homemade nitrogen-filled glovebox55 using a BAS 100B/W Electrochemical Analyzer (Bioanalytical System, West Lafayette, IN). The oxygen concentration inside the dry box was constantly monitored by a Series 800 Oxygen Analyzer (Illinois Instruments, Johnsburg, IL) and was always below 1 ppm. A glassy carbon (GC) macrodisk electrode from Cypress (Cypress Systems, Lawrence, KS), having an effective electrode area of 0.0299 cm2, or a 1 µm diameter Pt microelectrode were used as the working electrodes. Prior to each experiment, the working electrode was polished with 0.3 µm Alumina (Buehler, Lake Bluff, IL) on a clean polishing cloth (Buehler), sequentially rinsed with distilled water and acetone and finally dried with lint free tissue paper. A Pt wire was used as a counter electrode and a Ag wire sealed in a frit containing BMIMBF4 was used as a quasi-reference electrode (QRE). Measurement of Changes in Hydrogen Ion Concentration. The change of acidity of the ionic liquid solution (4 mM TCNQ + 7% (v/v) water) upon irradiation at 425 nm for 20 min was monitored by measurement of the apparent “pH” obtained with a glass electrode (Metrohm Model 744 pH electrode, Herisau, Switzerland). The apparent “pH” value of the same ionic liquid solution (4 mM TCNQ + 7% (v/v) water), but without irradiation, was also recorded as a control. The apparent “pH” values were recorded when the value becomes stable. Physical Characterization Procedures. The BMIMBF4 water content was determined by Karl Fischer titration (831 KF Coulometer, Metrohm). BMIMBF4 NMR investigations were undertaken at 20 °C (magnetic field strength of 7.04 T) with a Varian Unity-Plus 300 NMR Spectrometer. The chemical shifts for 11B and 19F were referenced to tetraborate and CFCl3 respectively. Spectral shimming was achieved via free induction. Viscosity measurements on BMIMBF4 were undertaken with an Anton Paar AMVn automated micro viscometer. Scanning Electron Microscopy (SEM) measurements were made with a JEOL 6300F Field Emission Gun (FEG) scanning electron microscope with a WinEDS energy dispersive X-ray (EDAX) analysis system. AgTCNQ samples photocrystallized onto glass, ITO or a membrane filter, were sputter coated with 10 nm Pt (Dynavac SC150 Sputter Coater) before imaging, to increase the production of secondary electrons. Infrared spectra were obtained with a Bruker IFS Equinox Fourier transform IR system equipped with a Golden GateTM single bounce diamond Micro-ATR (Attenuated Total Reflectance) cell. Raman spectra were collected with a Renishaw RM 2000 Raman spectrograph and microscope using a laser strength of 18 mW at a wavelength of 780 nm. Electronic spectra were obtained with a Varian Cary 5 UV-visible spectrophotometer (Varian OS2 software) using a 1 cm path length cell under benchtop laboratory conditions. Results and Discussion Preliminary Studies. The room temperature ionic liquid BMIMBF4 was chosen for use in this study because of the much (76) Ruether, T.; Hultgren, V. M.; Timko, B. P.; Bond, A. M.; Jackson, W. R.; Wedd, A. G. J. Am. Chem. Soc. 2003, 125, 10133. J. AM. CHEM. SOC.

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Figure 2. SEM images of AgTCNQ formed on a membrane substrate after 5 min irradiation (λ) 380 nm) of a liquid film of BMIMBF4 containing 10% v/v benzyl alcohol and equimolar (4.8 mM) TCNQ and Ag+.

Figure 1. UV-visible spectrum of 3 µM TCNQ in BMIMBF4. (Inset)

Highlight of the visible region.

higher solubility of TCNQ than that found in other ionic liquids. Initially, photochemical properties of TCNQ in BMIMBF4 were probed by UV-visible absorption spectroscopy. The UV-visible spectrum for 3 µM TCNQ in BMIMBF4 (Figure 1) exhibits a strong adsorption band with λmax ) 399 nm with a molar adsorption coefficient of 6.5 × 104 M-1 cm-1.This band resembles that reported in an organic solvent42,77-79 and hence may be assigned to the well-understood 1Ag f 1B3u transition for TCNQ. The weak absorbance, over the wavelength range of 700-800 nm (Figure 1, inset), is attributed to the absorbance from trace amounts of TCNQ- present and is assigned to the TCNQ- 2B2 g f 2B1u transition.77-79 UV-visible absorption spectra obtained in BMIMBF4 containing equimolar (3 µM) TCNQ and a Ag salt and 10% v/v benzyl alcohol mixture do not exhibit new bands at wavelengths over the range 200-800 nm attributable to a Ag-TCNQ charge transfer complex being formed in the ground state. In a control experiment, irradiation at λ) 380 nm of an ionic liquid drop containing the Ag salt showed no formation of Ag metal, which could subsequently react spontaneously with neutral TCNQ to form AgTCNQ. Storage of an equimolar (4.8 mM) mixture of TCNQ and Ag salt in the dark and normal laboratory light conditions for 24 h leads to no visual detection of blue AgTCNQ solid, but a slight color change from yellow to yellow-green was observed. These results are essentially in accordance with those reported in acetonitrile.42 Photocrystallization of AgTCNQ in BMIMBF4 in the Presence of the Sacrificial Electron Donor Benzyl Alcohol. A

wavelength of 380 nm, which is slightly less than λmax, was chosen for the excitation of TCNQ in initial photocrystallization experiments. To achieve photoreduction of TCNQ to TCNQand subsequent AgTCNQ formation requires the occurrence of a counter oxidization reaction. Benzyl alcohol was introduced into the system as a sacrificial electron donor since this has been previously shown to be oxidized to benzaldehyde in an organic solvent when irradiated by solar light in the presence of electron acceptors, such as polyoxometalates76 or TCNQ.42 Irradiation at λ) 380 nm for 5 min of a liquid film of BMIMBF4 containing 10% v/v benzyl alcohol as well as equimolar (4.8 (77) Lowitz, D. A. J. Chem. Phys. 1967, 46, 4698. (78) Yamagishi, A.; Sakamoto, M. Bull. Chem. Soc. Jpn. 1974, 49, 2152. (79) Liu, S. G.; Liu, Y. Q.; Wu, P. J.; Zhu, D. B.; Tien, H.; Chen, K. C. Thin Solid Films 1996, 289, 300. 4282

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Figure 3. SEM images of AgTCNQ formed on a membrane substrate after 30 min irradiation (λ) 380 nm) of a liquid film of BMIMBF4 containing 10% v/v benzyl alcohol and equimolar (4.8 mM) TCNQ and Ag+.

mM) TCNQ and Ag+ rapidly leads to visual detection of a blue spot in the yellow liquid film, which turns to dark blue with extended irradiation times. This is consistent with the photoreduction of TCNQ to TCNQ- and subsequent formation of blue AgTCNQ. The quantitative loss of benzyl alcohol and generation of benzaldehyde was confirmed by gas chromatography. A drop of blue suspension from the middle of the blue spot was then transferred to a solid substrate (membrane filter) surface and prepared for SEM imaging, according to the procedure described in the experimental section. SEM images after 5 min irradiation (Figure 2a) reveal the formation of a mixture of an extensive nanocrystal network consisting of very long nanowires and nanoparticles. Images taken at higher magnification (Figure 2b) imply that these wires are derived from clusters of nanoparticles and that they have rather rough surfaces. These images represent an early growth stage of the AgTCNQ nanowire formation. Elemental EDAX analysis shows both the nanowires and the nanoparticles contain Ag, C, and N, as expected if both wires and nanoparticles are AgTCNQ. After an increase for the irradiation time to 30 min, an extensive network of extremely long nanowires is formed (Figure 3a), with some nanowires exceeding a millimeter length and hence the SEM detection limit. These nanowires typically have a fairly uniform width of 30 to 50 nm, but a few thicker ones exhibit widths of up to 100 nm. The variation in width is attributed to the fusion or dissolution of nanowires at late growth stages. Inspection at higher magnification (Figure 3b) shows that most nanowires are smooth, even in width, and have a quadrangular cross section. Some nanoparticles and/or nanorods are also observed among the nanowire network, possibly derived from breakage of long nanowires. Raman spectra of photocrystallized AgTCNQ nanowires (Figure 4a) exhibit the four expected characteristic principle vibration modes at 1204 cm-1 (CdCH bending), 1385 cm-1 (C-CN wing stretching), 1605 cm-1 (CdC stretching), and 2211 cm-1 (C-N stretching). Comparison with the Raman

Photoinduced Oxidation of Water to Oxygen

Figure 4. Raman spectra of (a) photocrystallized AgTCNQ nanowires and (b) TCNQ crystals.

Figure 5. SEM images of AgTCNQ formed on a membrane substrate after 1 h irradiation (λ) 380 nm) of a liquid film of BMIMBF4 containing equimolar (4.8 mM) TCNQ and Ag+.

spectrum of neutral TCNQ (Figure 4b) reveals that the two principle vibration modes of TCNQ at 1455 cm-1(C-CN wing stretching) and 2227 (C-N stretching) are red-shifted by 70 cm-1 and 16 cm-1 respectively in AgTCNQ. The decrease in the vibrational energy is consistent with that obtained from other metal-TCNQ compounds and can be attributed to the charge transfer between atomic Ag and free TCNQ, when TCNQ is reduced to TCNQ- to form AgTCNQ.80 IR spectra further confirm the formation of AgTCNQ by exhibiting the expected characteristic bands at 2197, 2183, 2160, 1504 and 822 cm-1.14 Photocrystallization of AgTCNQ in BMIMBF4 in the Absence of a Deliberately Added Sacrificial Electron Donor.

Intriguingly, photocrystallization of AgTCNQ in BMIMBF4 under open bench laboratory conditions also could be achieved even in the absence of a deliberately added electron donor such as benzyl alcohol. Thus, irradiation at λ ) 380 nm for 1 h of a liquid film of BMIMBF4 containing equal molar (4.8 mM) TCNQ and Ag+, surprisingly, also leads to visual detection of a blue/green spot in the yellow liquid film. Again, with increasing irradiation time, the spot becomes dark blue. Transferal of a drop of the blue suspension to the membrane filter substrate, followed by SEM imaging, reveals that the blue solid formed in this way consists of numerous nanoparticles (Figure 5a, b) having diameters of ca. 30-50 nm. Elemental analysis using EDAX confirms the presence of Ag, C, and N, which is consistent with AgTCNQ. Raman spectra confirmed the formation of AgTCNQ by the presence of band at 1204, 1385, 1604, and 2209 cm-1.80 A small quantity of residual unwashed BMIMBF4 also remains between some nanoparticles (Figure 5). A distinctly different morphology is achieved if the suspension from the middle of blue spot is transferred to a glass or an (80) Kamitsos, E. I.; Tzinis, C. H.; Risen, W. M. Solid State Commun. 1982, 42, 561.

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Figure 6. SEM images of AgTCNQ formed on a ITO glass substrate after 1 h irradiation (λ ) 380 nm) of a liquid film of BMIMBF4 containing equimolar (4.8 mM) TCNQ and Ag+ followed by 24 h of incubation in the dark.

ITO substrate (instead of to a membrane filter substrate that rapidly soaks up the ionic liquid) then stored in dark for 24 h. Figure 6 shows SEM images obtained after 30 min irradiation followed by 24 h incubation in the dark. Exceptionally long nanowires are now detected over the entire substrate surface. The higher magnification images reveal that most of the nanowires have diameters lying in the range of 40-50 nm. Images at smaller magnification (not shown) reveal that these nanowires are extremely long (mm or greater and exceed the upper detection limit of the SEM instrument). A small number of isolated nanoparticles were also observed among the network of nanowires. Physical characterization of material formed in this manner by EDAX elemental analysis, IR spectroscopy and Raman spectroscopy confirm the formation of AgTCNQ. Mechanism of AgTCNQ Formation and Modified Properties in Ionic Liquids That Facilitate Oxidation of Water. Formation of extremely long AgTCNQ nanowires, which

might be coiled, is both fundamentally interesting and potentially of technological importance. The exceptional length may be attributed to the anisotropic growth of the AgTCNQ itself and the nanostructured anisotropic environment in ionic liquids, which favors the production of one-dimensional structures.59,62,74 Scheme 1 provides a cartoon form of presentation of the processes hypothesized to lead to the AgTCNQ nanowire formation. Irradiation gives rise to photoexcitation of TCNQ. The oxidation of the sacrificial electron donor such as benzyl alcohol to benzaldehyde then leads to formation of reduced TCNQ- anion (Scheme 1a). TCNQ has only a very short photoexcited-state lifetime. This is attributed to dicyanomethylene units in TCNQ, which dissipate excited-state energy through a free-rotor effect.81 Consequently, interaction with benzyl alcohol in the photoexcited state may be assumed to be highly favorable. AgTCNQ is then formed via reaction between TCNQ- and Ag+, and precipitated slowly in the form of small clusters (or nuclei) through homogeneous nucleation under conditions of supersaturation (Scheme 1b). A continuous supply of AgTCNQ enables these nuclei to serve as seeds for extended highly anisotropic growth along the columnar stack of TCNQ to form a nanorod (Scheme 1c). Continuation of irradiation provides more AgTCNQ, which allows each end of the rod to grow at a rate which is controlled by very slow diffusion in the highly viscous BMIMBF4. The process is assumed to be directed by the anisotropic nanostructure of the ionic liquid, to produce very long nanowires (Scheme 1d). In principle, the growth of the nanowires is endless, provided that AgTCNQ is constantly (81) Hixson, S. S.; Mariano, P. S.; Zimmerman, H. E. Chem. ReV. 1973, 73, 531. J. AM. CHEM. SOC.

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Zhao and Bond

Scheme 1. Schematic Representation of the Photocrystallization and Growth of AgTCNQ Nanowirea

a (a) Photoreduction of TCNQ to TCNQ-, and counter photooxidation of a sacrificial electron donor such as benzyl alcohol or water, (b) formation of AgTCNQ nuclei via homogeneous nucleation, (c) formation and growth of nanorod at both ends at a diffusion controlled rate, (d) formation of a nanowire of AgTCNQ.

supplied from the bulk ionic liquid solution and other conditions remain uninterrupted. In summary, AgTCNQ formation in the presence of a deliberately added electron donor benzyl alcohol in BMIMBF4 can be summarized by eqs 1 and 2. 2TCNQ(BMIMBF4) + hV

C6H5CH2OH(BMIMBF4) 98 2TCNQ(BMIMBF + 4) + C6H5CHO(BMIMBF4) + 2H(BMIMBF 4) + + 2Ag(BMIMBF f AgTCNQ(solid) 2TCNQ(BMIMBF 4) 4)

(1) (2)

The photocrystallization of AgTCNQ in BMIMBF4 in the absence of a deliberately added sacrificial electron donor was unexpected. The progressive formation of a large quantity of AgTCNQ materials upon irradiation indicates that the supply of this electron donor is abundant and steady. Given the composition of the system, possible counter reactions associated with photoreduction of TCNQ in BMIMBF4 considered were oxidation of Ag+ to Ag2+, oxidation of the constituent ions in the ionic liquid itself, oxidation of trace acetonitrile present in the Ag salt, or oxidation of impurities such as adventitious water known to be present in the ionic liquid under open bench laboratory conditions. The oxidation of BF4- was ruled out because of its well-known excellent chemical stability, which is demonstrated, for example, by the wide voltammetric potential window (4.1 V at a Pt electrode).53,54,56-58 Acetontrile also is a very stable and difficult to oxidize solvent, which has a potential window in excess of 6 V, provided that suitable supporting electrolytes are added.82 Thus acetonitrile oxidation also is highly unlikely. Oxidation of Ag+ to Ag2+ or oxidation of the BMIM+ cation also is very difficult to achieve. In view of the difficulty of oxidizing the species mentioned above and known to be present, we explored the possible role of impurities with respect to the counter oxidation process. BMIMBF4 can contain a range of impurities that are derived from the synthetic processes or decomposition of the constituent cation or anion. Halides are a common impurity arising from the synthesis. The manufacturer’s certificate of purity advises that the BMIMBF4 sample used in this study contains